The present invention relates generally to forced convection heat exchangers and, more particularly, to kilns that are heated by banks of finned pipes.
Lumber which has recently been cut contains a relatively large percentage of water and is referred to as green lumber. Prior to being used in construction or other applications which demand good grades of lumber, the green lumber must be dried. Drying removes a large amount of water from the lumber and significantly reduces the potential for the lumber to become warped or cracked.
Although lumber may be dried in the ambient air, kiln drying accelerates and provides increased control over the drying process. In kiln drying, a charge of lumber that typically consists of two or more rectangular stacks of lumber is placed in a kiln chamber. A typical kiln chamber is a generally rectangular building that can be at least partially sealed to control the amount of air that is introduced to and exhausted from the kiln chamber.
In one type of kiln, steam from a steam boiler is forced through banks of finned pipes that extend through the kiln chamber. The finned pipes heat air that is circulated within the kiln chamber by fans. The heated air flows along a flow path that extends through the charge of lumber. Conventional kilns and other types of drying apparatus that include heated pipes for heating a flow of air are disclosed in U.S. Pat. Nos. 1,513,639; 1,562,763; 1,774,208; 1,995,675 and 3,477,139. It is conventional for the pipes to be uniformly arranged so that they cause large blockage and resistance effects to the flow of air being circulated.
In conventional kilns of the type in which air is forced along a flow path that extends across finned pipes and through a charge of lumber for drying, the resistance to flow along the flow path that is caused by turns in the flow path and the finned pipes disadvantageously reduces the speed of the flow and the speed at which the charge can be dried. The reduction in the speed of the flow along the flow path can disadvantageously result in ununiform drying of the charge. The reduction in drying speed can be disadvantageous since mill production depends upon the ability to dry lumber at a sufficient rate so that production need not be slowed to allow for the drying process. The resistance to flow along the flow path also requires significant pressure increases to maintain the flow rate; therefore, the kiln fans, which force the heated air to flow along the flow path, must work excessively, which is disadvantageous. Operating the fans of a kiln consumes energy that adds to the cost of producing quality lumber. Of course it is advantageous to lower the cost of producing quality lumber. Whereas some conventional kilns can be characterized as being efficiently operated and able to dry lumber at a sufficient rate, there is always a demand for new kilns and kiln-related structures that can be even more efficiently operated, and that facilitate the drying of lumber at a sufficient rate.
The present invention solves the above and other problems by providing improved forced convection heat exchangers that can be used, for example, in kiln chambers. More specifically and according to one aspect of the present invention, a kiln system has one or more air moving devices for circulating air along a flow path that passes across one or more pipe banks and through a charge of lumber that is positioned in the interior space of a kiln chamber. The pipe bank(s) are advantageously arranged for limiting the resistance to flow along the flow path, for promoting mixing of the air flowing along the flow path, and for functioning as guide vanes for contributing to the defining of one or more turns in the flow path.
In accordance with one aspect of the present invention, each pipe bank has multiple finned pipes that extend parallel to one another and through which a heated medium flows. Each pipe bank is positioned in the flow path so that air circulated by the air moving device(s) flows exteriorly across the finned pipes and heat is transferred from the finned pipes to the circulated air. Each finned pipe has an elongate portion having opposite ends and extending in a longitudinal direction between the opposite ends. The longitudinal direction is at least generally perpendicular to the portion of the flow path that is proximate the finned pipe.
In accordance with one aspect of the present invention, a minimum distance defined between a pipe bank and any of the finned pipes that are not part of the pipe bank is substantially greater than the minimum distance defined between adjacent finned pipes in the pipe bank in an end elevation view of the elongate portions of the finned pipes. In addition, the finned pipes of the pipe bank are arranged at least generally in a line in the end elevation view. Further, the pipe bank is arranged and any minimum distance defined between adjacent finned pipes of the pipe bank is sufficiently small in the end elevation view so that the pipe bank functions as a guide vane for contributing to the defining of a turn in the flow path, with the turn at least generally following the line defined by the finned pipes of the pipe bank in the end elevation view.
In accordance with one aspect of the present invention, the pipe banks are spaced apart from one another and arranged side by side with respect to one another so that at least a portion of the flow path extends between adjacent ones of the pipe banks. The minimum distance defined between the adjacent pipe banks is substantially greater than the minimum distance defined between adjacent finned pipes in the same pipe bank in the end elevation view of the elongate portions of the finned pipes. Adjacent pipe banks are arranged to cooperatively function as guide vanes for contributing to the defining of a turn in the flow path, with the turn at least generally following the lines that are defined by the arrangement of the finned pipes of the adjacent pipe banks in the end elevation view.
In accordance with one aspect of the present invention, the adjacent pipe banks are at approximately the same position along the flow path.
In accordance with one aspect of the present invention, each of the lines that are defined by the arrangement of the finned pipes in the end elevation view is at least generally straight. Alternatively, each of the lines that are defined by the arrangement of the finned pipes in the end elevation view is curved. More specifically, each of the pipe banks at least generally defines concave and convex shapes in the end elevation view, with the concave shapes at least generally facing the center of the chamber interior space.
In accordance with one aspect of the present invention, the minimum distance defined between the adjacent pipe banks is greater than approximately one half the length of the line defined by the arrangement of the finned pipes of either of the adjacent pipe banks in the end elevation view.
In accordance with one aspect of the present invention, the surface of each pipe bank is rough in the direction of flow along the flow path, so that a turbulent boundary layer is produced along the surface of each of the pipe banks. The turbulent boundary layer advantageously enhances mixing and convective heat transfer. Additionally, some of the flow that passes generally along the flow path passes between adjacent finned pipes of the same pipe bank, so that mixing and convective heat transfer are advantageously enhanced.
In accordance with one aspect of the present invention, one of the pipe banks of the adjacent pipe banks is longer than the other of the pipe banks of the adjacent pipe banks in the end elevation view.
In accordance with one aspect of the present invention, each of the finned pipes has a maximum outer diameter, and any minimum distance defined between adjacent finned pipes of the same pipe bank is less than approximately one half the maximum outer diameter. Additionally, the minimum distance defined between adjacent pipe banks is greater than approximately the maximum outer diameter.
In accordance with one aspect of the present invention, at least a first one of the pipe banks has a predetermined number of the finned pipes, each of the finned pipes of the first pipe bank has approximately the same maximum outer diameter, and the line defined by the arrangement of the finned pipes of the first pipe bank extends between the opposite ends of the first pipe bank in the end elevation view and has a length of approximately the product of the maximum outer diameter and the predetermined number. According to this aspect of the present invention, the minimum distance defined between the first pipe bank and the closest adjacent pipe bank is greater than approximately one half the length of the line that is defined by the arrangement of the finned pipes of the first pipe bank in the end elevation view.
In accordance with one aspect of the present invention, a partition is positioned in the kiln chamber and generally separates the chamber interior space into an upper portion that is positioned above the partition and a lower portion that is positioned below the partition. The lower portion of the chamber interior space is for receiving the charge of lumber for drying. The air moving device(s) are positioned in the upper portion of the chamber interior space and the flow path extends around the partition and through the upper and lower portions of the chamber interior space. The kiln chamber has structure that define the chamber interior space and the partition has opposite first and second edges that extend in the longitudinal direction and are spaced apart from the structure. A first gap that extends in the longitudinal direction is defined between the first edge of the partition and the structure. Similarly, a second gap that extends in the longitudinal direction is defined between the second edge of the partition and the structure. A single pipe bank or at least a first pair of adjacent pipe banks is positioned in the first gap, and/or a single pipe bank or at least a second pair of adjacent pipe banks is positioned in the second gap.
In accordance with one aspect of the present invention, the kiln chamber can be more generally characterized as a chamber or shell of a heat exchanger, and the finned pipes can be replaced with pipes, or the like, without fins, or the like. The shell-side medium is not limited to air. Additionally, the shell of the heat exchanger is not required to accommodate fans or other types of means for moving the shell-side medium. Likewise, the shell of the heat exchanger is not required to accommodate a charge of lumber or any other type of workpiece. However, the pipes are advantageously positioned in banks that are disposed at least generally as described above.
In accordance with one aspect of the present invention, the pipes or finned pipes are arranged to advantageously: promote mixing of the medium flowing externally to the pipes or finned pipes so that heat transfer is enhanced, function as one or more guide vanes for contributing to the turning of the flow path of the externally flowing medium, and limit the resistance to flow along the flow path, so that the velocity of the flow can be increased.
These and other aspects and advantages of the present invention will be apparent to those or ordinary skill in the art in view of this disclosure.